Let x0 of type ι be given.
Let x1 of type ι → ι be given.
Let x2 of type ι be given.
Let x3 of type ι be given.
Assume H0:
x2 ∈ Pi x0 (λ x4 . x1 x4).
Apply SepE2 with
prim4 (lam x0 (λ x4 . prim3 (x1 x4))),
λ x4 . ∀ x5 . x5 ∈ x0 ⟶ ap x4 x5 ∈ x1 x5,
x2,
x3.
The subproof is completed by applying H0.